Active pulse blood constituent monitoring

ABSTRACT

A blood glucose monitoring system is disclosed which provides for inducing an active pulse in the blood volume of a patient. The induction of an active pulse results in a cyclic, and periodic change in the flow of blood through a fleshy medium under test. By actively inducing a change of the blood volume, modulation of the volume of blood can be obtained to provide a greater signal to noise ratio. This allows for the detection of constituents in blood at concentration levels below those previously detectable in a non-invasive system. Radiation which passes through the fleshy medium is detected by a detector which generates a signal indicative of the intensity of the detected radiation. Signal processing is performed on the electrical signal to isolate those optical characteristics of the electrical signal due to the optical characteristics of the blood.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to noninvasive systems for monitoringblood glucose and other difficult to detect blood constituentconcentrations, such as therapeutic drugs, drugs of abuse,carboxyhemoglobin, Methemoglobin, and cholesterol.

2. Description of the Related Art

In the past, many systems have been developed for monitoring bloodcharacteristics. For example, devices have been developed which arecapable of determining such blood characteristics as blood oxygenation,glucose concentration, and other blood characteristics. However,significant difficulties have been encountered when attempting todetermine blood glucose concentration accurately using noninvasive bloodmonitoring systems such as by means of spectroscopic measurement.

The difficulty in determining blood glucose concentration accurately maybe attributed to several causes. One of the significant causes is thatblood glucose is typically found in very low concentrations within thebloodstream (e.g., on the order of 100 to 1,000 times lower thanhemoglobin) so that such low concentrations are difficult to detectnoninvasively, and require a very high signal-to-noise ratio.Additionally, with spectroscopic methods, the optical characteristics ofglucose are very similar to those of water which is found in a very highconcentration within the blood. Thus, where optical monitoring systemsare used, the optical characteristics of water tend to obscure thecharacteristics of optical signals due to glucose within thebloodstream. Furthermore, since each individual has tissue, bone andunique blood properties, each measurement typically requires calibrationfor the particular individual.

In an attempt to accurately measure blood glucose levels within thebloodstream, several methods have been used. For example, one methodinvolves drawing blood from the patient and separating the glucose fromthe other constituents within the blood. Although fairly accurate, thismethod requires drawing the patient's blood, which is less desirablethan noninvasive techniques, especially for patients such as smallchildren or anemic patients. Furthermore, when blood glucose monitoringis used to control the blood glucose level, blood must be drawn three tosix times per day, which may be both physically and psychologicallytraumatic for a patient. Other methods contemplate determining bloodglucose concentration by means of urinalysis or some other method whichinvolves pumping or diffusing body fluid from the body through vesselwalls or using other body fluids such as tears or sweat. However, suchan analysis tends to be less accurate than a direct measurement ofglucose within the blood, since the urine, or other body fluid, haspassed through the kidneys (or skin in the case of sweat). This problemis especially pronounced in diabetics. Furthermore, acquiring urine andother body fluid samples is often inconvenient.

As is well known in the art, different molecules, typically referred toas constituents, contained within the medium have different opticalcharacteristics so that they are more or less absorbent at differentwavelengths of light. Thus, by analyzing the characteristics of thefleshy medium containing blood at different wavelengths, an indicationof the composition of the blood in the fleshy medium may be determined.

Spectroscopic analysis is based in part upon the Beer-Lambert law ofoptical characteristics for different elements. Briefly, Beer-Lambert'slaw states that the optical intensity of light through any mediumcomprising a single substance is proportional to the exponent of theproduct of path length through the medium times the concentration of thesubstance within the medium times the extinction coefficient of thesubstance. That is,

    I=I.sub.o e.sup.-(pl·c·ε)        (1)

where pl represents the path length through the medium, c represents theconcentration of the substance within the medium, ε represents theabsorption (extinction) coefficient of the substance and I_(o) is theinitial intensity of th light from the light source. For optical mediawhich have several constituents, the optical intensity of the lightreceived from the illuminated medium is proportional to the exponent ofthe path length through the medium times the concentration of the firstsubstance times the optical absorption coefficient associated with thefirst substance, plus the path length times the concentration of thesecond substance times the optical absorption coefficient associatedwith the second substance, etc. That is,

    I=I.sub.o e.sup.-(pl·c.sbsp.1.sup.·ε.sbsp.1.sup.+pl.multidot.c.sbsp.2.sup.·ε.sbsp.2.sup.+etc.)      (2)

where ε_(n) represents the optical absorption (extinction) coefficientof the n^(th) constituent and c_(n) represents the concentration of then^(th) constituent.

SUMMARY OF THE INVENTION

Due to the parameters required by the Beer-Lambert law, the difficultiesin detecting glucose concentration arise from the difficulty indetermining the exact path length through a medium (resulting fromtransforming the multi-path signal to an equivalent single-path signal),as well as difficulties encountered due to low signal strength resultantfrom a low concentration of blood glucose. Path length through a mediumsuch as a fingertip or earlobe is very difficult to determine, becausenot only are optical wavelengths absorbed differently by the fleshymedium, but also the signals are scattered within the medium andtransmitted through different paths. Furthermore, as indicated by theabove equation (2), the measured signal intensity at a given wavelengthdoes not vary linearly with respect to the path length. Therefore,variations in path length of multiple paths of light through the mediumdo not result in a linear averaging of the multiple path lengths. Thus,it is often very difficult to determine an exact path length through afingertip or earlobe for each wavelength.

In conventional spectroscopic blood constituent measurements, such ablood oxygen saturation, light is transmitted at various wavelengthsthrough the fleshy medium. The fleshy medium (containing blood)attenuates the incident light and the detected signal can be used tocalculate certain saturation values. In conventional spectroscopic bloodconstituent measurements, the heart beat provides a minimal modulationto the detected attenuated signal in order to allow a computation basedupon the AC portion of the detected signal with respect to the DCportion of the detected signal, as disclosed in U.S. Pat. No. 4,407,290.This AC/DC operation normalizes the signal and accounts for variationsin the pathlengths, as well understood in the art.

However, the natural heart beat generally provides approximately a 1-10%modulation (AC portion of the total signal) of the detected signal whenlight is transmitted through a patient's digit or the like. That is, thevariation in attenuation of the signal due to blood may be only 1% ofthe total attenuation (other attenuation being due to muscle, bone,flesh, etc.). In fact, diabetes patients typically have even lowermodulation (e.g., 0.01-0.1%). Therefore, the attenuation variation (ACportion of the total attenuation) due to natural pulse can be extremelysmall. In addition, the portion of the pulse modulation which is due toglucose is roughly only 9% of the pulse (approximately 1/11) at awavelength of 1330-1340 nm where glucose absorbs effectively.Furthermore, to resolve glucose from 5 mg/dl to 1005 mg/dl in incrementsor steps of 5 mg/dl, requires resolution of 1/200 of the 9% of themodulation which is due to glucose. Accordingly, by way of threedifferent examples--one for a healthy individual, one for a diabeticwith a strong pulse, and one for a diabetic with a weak pulse--forabsorption at 1330 nm, the system would require resolution as follows.

EXAMPLE 1

Healthy individuals where natural pulse provides attenuation modulationof 1% at 1330 nm:

a. Natural modulation due to pulse is approximately 1% (1/100).

b. Portion of natural modulation due to glucose is approximately 9%(1/11).

c. To resolve glucose from 5-1005 mg/dl requires resolution of 1/200(i.e., there are 200, 5 mg/dl steps between 5 and 1005 mg/dl).

Required Total Resolution is product of a-c:

    1/100*1/11*1/200=1/220,000

EXAMPLE 2

Diabetic where natural pulse provides attenuation modulation of 0.1% at1330 nm

a. Natural modulation due to pulse approximately 0.1% (1/1000).

b. Portion of natural modulation due to glucose is approximately 9%(1/11)

c. To resolve glucose from 5-1005 mg/dl requires resolution of 1/200.

Required total resolution is product of a-c:

    1/1000*1/11*1/200=1/2,200,000

EXAMPLE 3

Diabetic where natural pulse provides attenuation modulation of 0.01%

a. Natural modulation due to pulse approximately 0.01% (1/10,000).

b. Portion of natural modulation due to glucose is approximately 9%(1/11).

c. To resolve glucose from 5-1005 mg/dl requires resolution of 1/200.

Required total resolution is product of a-c:

    1/10,000* 1/11* 1/200=1/22,000,000

As seen from the above three examples which provide the range ofmodulation typically expected among human patients, the total resolutionrequirements range from 1 in 220,000 to 1 in 22,000,000 in order todetect the attenuation which is due to glucose based on the naturalpulse for the three examples. This is such a small portion that accuratemeasurement is very difficult. In most cases, the noise accounts for agreater portion of the AC portion (natural modulation due to pulse) ofthe signal than the glucose, leaving glucose undetectable. Even withstate of the art noise reduction processing as described in U.S. patentapplication Ser. No. 08/249,690, filed May 26, 1994, signals may beresolved to a level of approximately 1/250,000. This is for an 18-bitsystem. With a 16-bit system, resolution is approximately 1/65,000. Inaddition, LEDs are often noisy such that even if resolution in thesystem is available to 1/250,000, the noise from the LEDs leave glucoseundetectable.

To overcome these obstacles, it has been determined that by activelyinducing a change in the flow of blood in the medium under test suchthat the blood flow varies in a controlled manner periodically,modulation can be obtained such that the portion of the attenuatedsignal due to blood becomes a greater portion of the total signal thanwith modulation due to the natural pulse. This leads to the portion oftotal attenuation due to glucose in the blood being a greater portion ofthe total signal. In addition, the signal can be normalized to accountfor factors such as source brightness, detector responsiveness, tissueor bone variation. Changes in blood flow can be induced in several ways,such as physically perturbing the medium under test or changing thetemperature of the medium under test. In the present embodiment, byactively inducing a pulse, a 10% modulation in attenuation (1/10 of thetotal attenuation) is obtained, regardless of the patient's naturalpulse modulation (whether or not the patient is diabetic). Accordingly,at 1330 nm with actively induced changes in blood flow, the resolutionrequired is 1/10*1/11*1/200 or 1/22,000 (where 1/10 is the active pulseattenuation modulation (the modulation obtained by induced blood flowchanges), 1/11 is the portion of the modulation due to glucose, and1/200 the resolution required to obtain glucose in 5 mg/dl incrementsfrom 5-1005 mg/dl). As will be understood from the discussion above,such resolution can be obtained, even in a 16 bit system. In addition,the resolution is obtainable beyond the noise floor, as describedherein.

In conventional blood constituent measurement through spectroscopy,perturbation of the medium under test has been avoided because oxygen(the most commonly desired parameter) is not evenly dispersed in thearterial and venous blood. Therefore, perturbation obscures the abilityto determine the arterial oxygen saturation because that venous andarterial blood become intermingled. However, glucose is evenly dispersedin blood fluids, so the mixing of venous and arterial blood andinterstitial fluids should have no significant effect on the glucosemeasurements. It should be appreciated that this technique will beeffective for any substance evenly dispersed in the body fluids (e.g.,blood, interstitial fluids, etc.).

One aspect of the present invention involves a system for non-invasivelymonitoring a blood constituent concentration in a living subject. Thesystem comprises a light source which emits radiation at a plurality ofwavelengths and an active pulse inducement device which, independent ofthe natural flow of blood in the fleshy medium, causes a periodic changein the volume of blood in the fleshy medium. An optical detectorpositioned to detect light which has propagated through the fleshymedium is configured to generate an output signal indicative of theintensity of the radiation after attenuation through the fleshy medium.A signal processor responds to the output signal to analyze the outputsignal to extract portions of the signal due to optical characteristicsof the blood to determine the concentration of the constituent withinthe subject's bloodstream.

In one embodiment, of the system further comprises a receptacle whichreceives the fleshy medium, the receptacle further having an inflatablebladder.

In one embodiment, the system has a temperature variation element in thereceptacle, the temperature variation element varies (e.g., increases)the temperature of the fleshy medium in order to induce a change (e.g.,increase) in the flow of blood in the fleshy medium.

Another aspect of the present invention involves a system fornon-invasively monitoring blood glucose concentration within a patient'sbloodstream. A light source emits optical radiation at a plurality offrequencies, and a sensor receives a fleshy medium of the patient, thefleshy medium having flowing blood. A fluid (e.g., blood andinterstitial fluids) volume change inducement device causes a cyclicchange in the volume of blood in the fleshy medium. An optical detectorpositioned to receive the optical radiation after transmission through aportion of the fleshy medium responds to the detection of the opticalradiation to generate an output signal indicative of the intensity ofthe optical radiation. A signal processor coupled to the detectorreceives the output signal, and responds to the output signal togenerate a value representative of the glucose concentration in theblood of the patient.

Yet another aspect of the present invention involves a method ofnon-invasively determining a concentration of a blood constituent. Themethod comprises a plurality of steps. Optical radiation is transmittedthrough a medium having flowing fluid, wherein the fluid has aconcentration of the fluid constituent. A periodic change in the volumeof the fluid in the medium is actively induced. The optical opticalradiation after transmission through at least a portion of the medium isdetected and a signal indicative of the optical characteristics of themedium is generated. The sigal is analyzed to determine theconcentration of the blood constituent. In one embodiment, the fluidconstituent comprises blood glucose.

A further aspect of the present invention involves a method of activelyvarying the attenuation of optical radiation due to blood in a fleshymedium. The method comprises a plurality of steps. Optical radiation istransmitted through the fleshy medium. A periodic change in the volumeof blood is actively influenced in the medium. The optical radiation isdetected after attenuation through the fleshy medium and an outputsignal indicative of the intensity of the attenuated signal isgenerated.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts an embodiment of a blood glucose monitor of the presentinvention.

FIG. 2 depicts an example of a physiological monitor in accordance withthe teachings of the present invention.

FIG. 2A illustrates an example of a low noise emitter current driverwith accompanying digital to analog converter.

FIG. 2B depicts an embodiment of FIG. 2 with added function fornormalizing instabilities in emitters of FIG. 2.

FIG. 2C illustrates a comparison between instabilites in selectedemitters.

FIG. 3 illustrates the front end analog signal conditioning circuitryand the analog to digital conversion circuitry of the physiologicalmonitor of FIG. 2.

FIG. 4 illustrates further detail of the digital signal processingcircuitry of FIG. 2.

FIG. 5 illustrates additional detail of the operations performed by thedigital signal processing circuitry of FIG. 2.

FIG. 6 illustrates additional detail regarding the demodulation moduleof FIG. 5.

FIG. 7 illustrates additional detail regarding the decimation module ofFIG. 5.

FIG. 8 represents a more detailed block diagram of the operations of theglucose calculation module of FIG. 5.

FIG. 9 illustrates the extinction coefficient versus wavelength forseveral blood constituents.

FIG. 10-12 depict one embodiment of a probe which can be used to inducean active pulse in accordance with the principals of the presentinvention.

FIG. 12 a depicts the embodiment of FIG. 12 in the inflated state.

FIG. 13 depicts an example of the an active pulse signal where themodulation is 10% of the entire attenuation through the finger.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 depicts one embodiment of a blood glucose monitor system 100 inaccordance with the teachings of the present invention. The glucosemonitor 100 of FIG. 1 has an emitter 110 such as light emitting diodesor a light with a filter wheel as disclosed in U.S. patent applicationSer. No. (08/479,164 entitled Blood Glucose Monitoring System, filed onthe same day as this application, and assigned to the assignee of thisapplication, which application is incorporated by reference herein.

The filter wheel with a broadband light is depicted in FIG. 1. Thisarrangement comprises a filter wheel 110A, a motor 110B, and a broadbandlight source 110C. Advantageously, this unit can be made relativelyinexpensively as a replaceable unit. The filter wheel is advantageouslymade accordance with U.S. patent application Ser. No. 08/486,798,entitled Optical Filter for Spectroscopic Measurement and Method ofProducing the Optical Filter, filed on the same date as thisapplication, and assigned to the assignee of this application, whichapplication is incorporated herein by reference.

The monitor system 100 has a detector 140, such as a photodetector. Theblood glucose monitor 100 also has a pressure inducing cuff 150 tophysically squeeze a digit 130 in order to periodically induce a "pulse"in the fluid (i.e., actively vary the flow of fluid) in a digit 130. Inother words, a device influences a change in the volume of blood in thedigit or other fleshy medium. A window 111 is positioned to allow lightfrom the emitter 110 to pass through the window 11 and transmit throughthe digit 130. This intentional active perturbation of the blood in thedigit or medium under test is further referred to herein as an "activepulse." The blood glucose monitor also has a display 160 which may beused to indicate such parameters as glucose concentration and signalquality. Advantageously, the blood glucose monitor also has a powerswitch 154, a start switch 156 and a trend data switch 158.

Other methods of inducing a pulse are also possible. For instance, thefleshy medium under test, such as the patient's digit, could beperturbed with a pressure device 152 (depicted in dotted lines in FIG.1). Other methods of inducing a pulse could be utilized such astemperature fluctuations or other physiological changes which result ina fluctuation (modulation) of blood volume through the fleshy medium.All external methods (as opposed to the natural heart beat) activelyvary the blood volume in the medium under test are collectively referredto herein as inducing an "active pulse." In the present embodiment, 10%modulation in the total attenuation is obtained through the activeinduction of a pulse. The 10% modulation is selected as a level ofminimal perturbation to the system. Too much perturbation of the mediumwill change the optical characteristics of the medium under test. Forinstance, with substantial modulation (e.g., 40-50%), the perturbationcould impact scattering within the medium under test differently fordifferent wavelengths, thus causing inacurate measurements.

The pressure device 152, the cuff 150 and the use of temperature toinduce a pulse in the fleshy medium are advantageous in that they can beused with minimal or no movement of the fleshy medium in the areathrough which light is transmitted. This is possible through inducingthe pulse at a location proximal or distal from the area receiving theincident light. The advantage of minimal movement is that movement inthe area of the fleshy medium under test causes variation in thedetected signal other than due to the varying fluid volume (e.g., bloodand interstitial fluid) flow. For instance, physical perturbation in thearea of light transmission can cause changes in the light coupling tothe medium under test resulting in variations in attenuation which arenot due to changes in fluid volume in the area of light transmission.These other variations comprise additional noise that should be removedfor accurate measurement.

FIG. 2-4 depict a schematic block diagram of the blood glucosemonitoring system 100 in accordance with the teachings of the presentinvention. FIG. 2 illustrates a general hardware block diagram. A sensor300 has multiple light emitters 301-305 such as LED's. In the presentembodiment, each LED 301-305 emits light at a different wavelength.

As well understood in the art, because Beer-Lambert's law contains aterm for each constituent which attenuates the signal, one wavelength isprovided for each constituent which is accounted for. For increasedprecision, the wavelengths are chosen at points where attenuation foreach particular constituent is the greatest and attenuation by otherconstituents is less significant. FIG. 9 depicts the extinctioncoefficient on a log scale vs. wavelength for principal bloodconstituents. The curve 162 represents the extinction coefficient foroxyhemoglobin; the curve 164 represents the extinction coefficient forhemoglobin; the curve 165 represents the extinction coefficient forcarboxyhemoglobin; and the curve 166 represents the extinctioncoefficient for water. Depicted on the same horizontal axis with adifferent vertical axis is a curve 168 which represents the extinctioncoefficient for glucose in body fluids. It should be noted that thecurve 168 is placed above the other curves and is greatly amplified, andtherefore is not to scale on the graph. If the glucose curve weregraphed on the same scale as the other constituents, it would simplyappear as flat line at `0` on the vertical axis in the wavelength rangefrom 900-1400 mm. The provision for a seperate vertical axis providesfor amplification in order to illustrate at which wavelengths glucoseattenuates the most in the range of interest. The vertical axis for theglucose curve 168 also represents a different value. In FIG. 9, thevertical axis for the curve 168 is in terms of the absolute transmissionon the following log scale:

    [log(log(average water))]-[log(log(6400 mg/dl glucose))]

However, for purposes of choosing appropriate wavelengths, the scale isof less significance that the points at which Glucose and the otherconstituents show good attenuation and the attenuation is not totallyobscured by other constituents in the medium.

In the present embodiment, advantageous wavelengths for the emitters301-305 (or to obtain with the filter wheel and signal processing) are660 nm (good attenuation hemoglobin), 905 nm (good attenuation fromoxyhemoglobin), 1270 nm (good attenuation by water, and littleattenuation by other constituents) 1330-1340 nm (good attenuation due toGlucose in the area of the graph labelled A of FIG. 9, not totallyobscured by the attenuation due to water), and 1050 nm (an additionalpoint for good attenuation from Glucose). The use of two wavelengths toaccount for glucose attenuation provides overspecification of theequations. Overspecification of the equations discussed below increasesresolution. Additional wavelengths to account for other constituentssuch as fats and proteins or others could also be included. Forinstance, an additional wavelength at 1100 nm could be added (goodattenuation from proteins) and 920 nm (good attenuation from fats).Another constituent often of interest is carboxyhemoglobin. A wavelengthfor carboxyhemoglobin is advantageously selected at 700-730 nm.

In addition to using multiple precise LEDs, an optical spectroscopicsystem for generating the optical characteristics over many wavelengthscan be used. Such a device is disclosed in U.S. patent application Ser.No. 08/479,169, entitled Blood Glucose Monitoring System, filed on thesame day as this application, and assigned to the assignee of thisapplication.

The sensor 300 further comprises a detector 320 (e.g., a photodetector),which produces an electrical signal corresponding to the attenuatedlight energy signals. The detector 320 is located so as to receive thelight from the emitters 301-305 after it has propagated through at leasta portion of the medium under test. In the embodiment depicted in FIG.2, the detector 320 is located opposite the LED's 301-305. The detector320 is coupled to front end analog signal conditioning circuity 330.

The front end analog signal conditioning circuitry 330 has outputscoupled to analog to digital conversion circuit 332. The analog todigital conversion circuitry 332 has outputs coupled to a digital signalprocessing system 334. The digital signal processing system 334 providesthe desired parameter as an output for a display 336. The display 336provides a reading of the blood glucose concentration.

The signal processing system also provides an emitter current controloutput 337 to a digital-to-analog converter circuit 338 which providescontrol information for emitter drivers 340. The emitter drivers 340couple to the emitters 301-305. The digital signal processing system 334also provides a gain control output 342 for the front end analog signalconditioning circuitry 330.

FIG. 2A illustrates a preferred embodiment for the emitter drivers 340and the digital to analog conversion circuit 338. The driver depicted inFIG. 2a is depicted for two LEDs coupled back-to-back. However,additional LEDs (preferably coupled back-to-back to conserveconnections) can be coupled to the D/A converter 325 through additionalmultiplexing circuitry (not shown). As depicted in FIG. 2A, the drivercomprises first and second input latches 321, 322, a synchronizing latch323, a voltage reference 324, a digital to analog conversion circuit325, first and second switch banks 326, 327, first and second voltage tocurrent converters 328, 329 and the LED emitters 301, 302 correspondingto the LED emitters 301-302 of FIG. 2.

The preferred driver depicted in FIG. 2A is advantageous in that much ofthe noise in the blood glucose system 100 of FIG. 2 is caused by the LEDemitters 301-305. Therefore, the emitter driver circuit of FIG. 2A isdesigned to minimize the noise from the emitters 301-305. The first andsecond input latches 321, 324 are connected directly to the DSP bus.Therefore, these latches significantly minimize the bandwidth (resultingin noise) present on the DSP bus which passes through to the drivercircuitry of FIG. 2A. The output of the first and second input latchesonly changes when these latches detect their address on the DSP bus. Thefirst input latch receives the setting for the digital to analogconverter circuit 325. The second input latch receives switching controldata for the switch banks 326, 327. The synchronizing latch accepts thesynchronizing pulses which maintain synchronization between theactivation of emitters 301, 302 (and the other emitters 303-305 notdepicted in FIG. 2a) and the analog to digital conversion circuit 332.

The voltage reference is also chosen as a low noise DC voltage referencefor the digital to analog conversion circuit 325. In addition, in thepresent embodiment, the voltage reference has an lowpass output filterwith a very low corner frequency (e.g., 1 Hz in the present embodiment).The digital to analog converter 325 also has a lowpass filter at itsoutput with a very low corner frequency (e.g., 1 Hz). The digital toanalog converter provides signals for each of the emitters 301, 302 (andthe remaining emitters 303-305, not depicted in FIG. 2a).

In the present embodiment, the output of the voltage to currentconverters 328, 329 are switched such that with the emitters 301, 302connected in back-to-back configuration, only one emitter is active anany given time. A refusal position for the switch 326 is also providedto allow the emitters 301 and 302 to both be off when one of the otheremitters 303-305 is on with a similar switching circuit. In addition,the voltage to current converter for the inactive emitter is switchedoff at its input as well, such that it is completely deactivated. Thisreduces noise from the switching and voltage to current conversioncircuitry. In the present embodiment, low noise voltage to currentconverters are selected (e.g., Op 27 Op Amps), and the feedback loop isconfigured to have a low pass filter to reduce noise. In the presentembodiment, the low pass filtering function of the voltage to currentconverter 328, 329 has a corner frequency just above the switching speedfor the emitters. Accordingly, the preferred driver circuit of FIG. 2a,minimizes the noise of the emitters 301, 302.

As represented in FIG. 2, the light emitters 301-305 each emits energywhich is absorbed by the finger 310 and received by the detector 320.The detector 320 produces an electrical signal which corresponds to theintensity of the light energy striking the photodetector 320. The frontend analog signal conditioning circuitry 330 receives the intensitysignals and filters and conditions these signals as further describedbelow for further processing. The resultant signals are provided to theanalog-to-digital conversion circuitry 332 which converts the analogsignals to digital signals for further processing by the digital signalprocessing system 334. The digital signal processing system 334 utilizesthe signals in order to provide blood glucose concentration. In thepresent embodiment, the output of the digital signal processing system334 provides a value for glucose saturation to the display 336.Advantageously, the signal processing system 334 also store data over aperiod of time in order to generate trend data and perform otheranalysis on the data over time.

The digital signal processing system 334 also provides control fordriving the light emitters 301-305 with an emitter current controlsignal on the emitter current control output 337. This value is adigital value which is converted by the digital-to-analog conversioncircuit 338 which provides a control signal to the emitter currentdrivers 340. The emitter current drivers 340 provide the appropriatecurrent drive for the emitters 301-305.

In the present embodiment, the emitters 301-305 are driven via theemitter current driver 340 to provide light transmission with digitalmodulation at 625 Hz. In the present embodiment, the light emitters301-305 are driven at a power level which provides an acceptableintensity for detection by the detector and for conditioning by thefront end analog signal conditioning circuitry 330. Once this energylevel is determined for a given patient by the digital signal processingsystem 334, the current level for the emitters is maintained constant.It should be understood, however, that the current could be adjusted forchanges in the ambient room light and other changes which would effectthe voltage input to the front end analog signal conditioning circuitry330. In the present invention, light emitters are modulated as follows:for one complete 625 Hz cycle for the first wavelength, the firstemitter 301 is activated for the first tenth of the cycle, and off forthe remaining nine-tenths of the cycle; for one complete 625 Hz secondwavelength cycle, the second light emitter 302 is activated for the onetenth of the cycle and off for the remaining nine-tenths cycle; for one625 Hz third wavelength cycle, the third light emitter 303 is activatedfor one tenth cycle and is off for the remaining nine-tenths cycle; forone 625 Hz fourth wavelength cycle, the fourth light emitter 304 isactivated for one tenth cycle and is off for the remaining nine-tenthscycle; and for one 625 Hz fifth wavelength cycle, the fifth lightemitter 305 is activated for one tenth cycle and is off for theremaining nine-tenths cycle. In order to receive only one signal at atime, the emitters are cycled on and off alternatively, in sequence,with each only active for a tenth cycle per 625 Hz cycle and a tenthcycle separating the active times.

The light signal is attenuated (amplitude modulated) by the blood (withthe volume of blood changing through cyclic active pulse in the presentembodiment) through the finger 310 (or other sample medium). In thepresent embodiment, the fingertip 130 is physiologically altered on aperiodic basis by the pressure device 150 (or the active pulse device)so that approximately 10% amplitude modulation is achieved. That is,enough pressure is applied to the fingertip 310 to evacuate a volume ofbody fluid such that the variation in the overall difference in opticalattenuation observed between the finger tip 310 when full of blood andthe finger tip 310 when blood is evacuated, is approximately 10%. Forexample, if the transmission of optical radiation through the fingertip310 is approximately 0.4%, then the fingertip 310 would have to bephysiologically altered to evacuate enough blood so that the attenuationof the fingertip having fluid evacuated would be on the order to 0.36%.FIG. 13 depicts an example of the an active pulse signal where themodulation is 10% of the entire attenuation through the finger. The 10%is obtained by varying the volume of blood enough to obtain the cyclicmodulation depicted in FIG. 13. As explained above, the 10% modulationis chosen as sufficient to obtain information regarding glucoseconcentrations, yet cause minimal perturbation to the system. Minimalperturbation is advantageous due to the optical variations caused byperturbing the system. The level of perturbation is advantageously belowa level that causes significant variations in optical properties in thesystem, which variations affect different wavelengths differently.

In one advantageous embodiment, physiological altering of the fingertip310 is accomplished by the application of periodic gentle pressure tothe patient's finger 310 with the pressure cuff 150 (FIG. 1). The finger310 could also be perturbed by the pressure device 152 (FIG. 1) or withtemperature.

The modulation is performed at a selected rate. A narrow band passfilter may then be employed to isolate the frequency of interest. In thepresent embodiment, the modulation obtained through influencing anactive pulse preferably occurs at a rate just above the normal heartrate (for instance, 4 Hz). In one embodiment, the system checks theheart rate and sets the active pulse rate such that it is above thenatural heart rate, and also away from harmonics of the natural pulserate. This allows for easy filtering with a very narrow band-pass filterwith a center frequency of at the selected active pulse rate (e.g., 4 Hzor the rate automatically selected by the system to be away from thefundamental natural heart rate frequency and any harmonics to thefundamental frequency). However, a frequency in or below the range ofnormal heart rate could also be used. Indeed, in one embodiment, thefrequency tracks the heart rate, in which case the active pulse operatesin conjunction with the natural pulse to increase the change in volumeof flow with each heart beat.

The attenuated (amplitude modulated) signal is detected by thephotodetector 320 at the 625 Hz carrier frequency for each emitter.Because only a single photodetector is used, the photodetector 320receives all the emitter signals to form a composite time divisionsignal. In the present embodiment, a photodetector is provided which isa sandwich-type photodetector with a first layer which is transparent toinfrared wavelengths but detects red wavelengths and a second layerwhich detects infrared wavelengths. One suitable photodetector is aK1713-05 photodiode made by Hamamatsu Corp. This photodetector providesfor detection by the infrared layer of a relatively large spectrum ofinfrared wavelengths, as well as detection of a large spectrum ofwavelengths in the red range by the layer which detects red wavelengths,with a single photodetector. Alternatively, multiple photodetectorscould be utilized for the wavelengths in the system.

The composite time division signal is provided to the front analogsignal conditioning circuitry 330. Additional detail regarding the frontend analog signal conditioning circuitry 330 and the analog to digitalconverter circuit 332 is illustrated in FIG. 3. As depicted in FIG. 3,the front end circuity 300 has a preamplifier 342, a high pass filter344, an amplifier 346, a programmable gain amplifier 348, and a low passfilter 350. The preamplifier 342 is a transimpedance amplifier thatconverts the composite current signal from the photodetector 320 to acorresponding voltage signal, and amplifies the signal. In the presentembodiment, the preamplifier has a predetermined gain to boost thesignal amplitude for ease of processing. In the present embodiment, thesource voltages for the preamplifier 342 are -15 VDC and +15 VDC. Aswill be understood, the attenuated signal contains a componentrepresenting ambient light as well as the component representing thelight at each wavelength transmitted by each emitter 301-305 as the casemay be in time. If there is light in the vicinity of the sensor 300other than from the emitters 301-305, this ambient light is detected bythe photodetector 320. Accordingly, the gain of the preamplifier isselected in order to prevent the ambient light in the signal fromsaturating the preamplifier under normal and reasonable operatingconditions.

The output of the preamplifier 342 couples as an input to the high passfilter 344. The output of the preamplifier also provides a first input347 to the analog to digital conversion circuit 332. In the presentembodiment, the high pass filter is a single-pole filter with a cornerfrequency of about 1/2-1 Hz. However, the corner frequency is readilyraised to about 90 Hz in one embodiment. As will be understood, the 625Hz carrier frequency of the emitter signals is well above a 90 Hz cornerfrequency. The high-pass filter 344 has an output coupled as an input toan amplifier 346. In the present embodiment, the amplifier 346 comprisesa unity gain transimpedance amplifier. However, the gain of theamplifier 346 is adjustable by the variation of a single resistor. Thegain of the amplifier 346 would be increased if the gain of thepreamplifier 342 is decreased to compensate for the effects of ambientlight.

The output of the amplifier 346 provides an input to a programmable gainamplifier 348. The programmable gain amplifier 348 also accepts aprogramming input from the digital signal processing system 334 on again control signal line 343. The gain of the programmable gainamplifier 348 is digitally programmable. The gain is adjusteddynamically at initialization or sensor placement for changes in themedium under test from patient to patient. For example, the signal fromdifferent fingers differs somewhat. Therefore, a dynamically adjustableamplifier is provided by the programmable gain amplifier 348 in order toobtain a signal suitable for processing.

The output of the programmable gain amplifier 348 couples as an input toa low-pass filter 350. Advantageously, the low pass filter 350 is asingle-pole filter with a corner frequency of approximately 10 Khz inthe present embodiment. This low pass filter provides anti-aliasing inthe present embodiment.

The output of the low-pass filter 350 provides a second input 352 to theanalog-to-digital conversion circuit 332. FIG. 3 also depicts additionaldetails of the analog-to-digital conversion circuit. In the presentembodiment, the analog-to-digital conversion circuit 332 comprises afirst analog-to-digital converter 354 and a second analog-to-digitalconverter 356. Advantageously, the first analog-to-digital converter 354accepts signals from the first input 347 to the analog-to-digitalconversion circuit 332, and the second analog to digital converter 356accepts signals on the second input 352 to the analog-to-digitalconversion circuitry 332.

In one advantageous embodiment, the first analog-to-digital converter354 is a diagnostic analog-to-digital converter. The diagnostic task(performed by the digital signal processing system) is to read theoutput of the detector as amplified by the preamplifier 342 in order todetermine if the signal is saturating the input to the high-pass filter344. In the present embodiment, if the input to the high pass filter 344becomes saturated, the front end analog signal conditioning circuits 330provides a `0` output. Alternatively, the first analog-to-digitalconverter 354 remains unused.

The second analog-to-digital converter 352 accepts the conditionedcomposite analog signal from the front end signal conditioning circuitry330 and converts the signal to digital form. In the present embodiment,the second analog to digital converter 356 comprises a single-channel,delta-sigma converter. This converter is advantageous in that it is lowcost, and exhibits low noise characteristics. In addition, by using asingle-channel converter, there is no need to tune two or more channelsto each other. The delta-sigma converter is also advantageous in that itexhibits noise shaping, for improved noise control. An exemplary analogto digital converter is an Analog Devices AD1877JR. In the presentembodiment, the second analog to digital converter 356 samples thesignal at a 50 Khz sample rate. The output of the second analog todigital converter 356 provides data samples at 50 Khz to the digitalsignal processing system 334 (FIG. 2).

The digital signal processing system 334 is illustrated in additionaldetail in FIG. 4. In the present embodiment, the digital signalprocessing system comprises a microcontroller 360, a digital signalprocessor 362, a program memory 364, a sample buffer 366, a data memory368, a read only memory 370 and communication registers 372. In thepresent embodiment, the digital signal processor 362 is an AnalogDevices AD 21020. In the present embodiment, the microcontroller 360comprises a Motorola 68HC05, with built in program memory. In thepresent embodiment, the sample buffer 366 is a buffer which accepts the50 Khz sample data from the analog to digital conversion circuit 332 forstorage in the data memory 368. In the present embodiment, the datamemory 368 comprises 32 KWords (words being 40 bits in the presentembodiment) of dynamic random access memory.

The microcontroller 360 is connected to the DSP 362 via a conventionalJTAG Tap line. The microcontroller 360 transmits the boot loader for theDSP 362 to the program memory 364 via the Tap line, and then allows theDSP 362 to boot from the program memory 364. The boot loader in programmemory 364 then causes the transfer of the operating instructions forthe DSP 362 from the read only memory 370 to the program memory 364.Advantageously, the program memory 364 is a very high speed memory forthe DSP 362.

The microcontroller 360 provides the emitter current control and gaincontrol signals via the communications register 372.

FIGS. 5-8 depict functional block diagrams of the operations of theglucose monitoring system 299 carried out by the digital signalprocessing system 334. The signal processing functions described beloware carried out by the DSP 362 in the present embodiment with themicrocontroller 360 providing system management. In the presentembodiment, the operation is software/firmware controlled. FIG. 5depicts a generalized functional block diagram for the operationsperformed on the 50 Khz sample data entering the digital signalprocessing system 334. As illustrated in FIG. 5, a demodulation, asrepresented in a demodulation module 400, is first performed.Decimation, as represented in a decimation module 402 is then performedon the resulting data. Then, the glucose concentration is determined, asrepresented in a Glucose Calculation module 408.

In general, the demodulation operation separates each emitter signalfrom the composite signal and removes the 625 Hz carrier frequency,leaving raw data points. The raw data points are provided at 625 Hzintervals to the decimation operation which reduces the samples by anorder of 10 to samples at 62.5 Hz. The decimation operation alsoprovides some filtering on the samples. The resulting data is subjectedto normalization (which essentially generates a normalized AC/DC signal)and then glucose concentration is determined in the Glucose Calculationmodule 408.

FIG. 6 illustrates the operation of the demodulation module 400. Themodulated signal format is depicted in FIG. 6. The pulses for the firstthree wavelengths of one full 625 Hz cycle of the composite signal isdepicted in FIG. 6 with the first tenth cycle being the active firstemitter light plus ambient light signal, the second tenth cycle being anambient light signal, the third tenth cycle being the active secondemitter light plus ambient light signal, and the fourth tenth cyclebeing an ambient light signal, and so forth for each emitter. Thesampling frequency is selected at 50 Khz so that the single full cycleat 625 Hz described above comprises 80 samples of data, eight samplesrelating to the first emitter wavelength plus ambient light, eightsamples relating to ambient light, eight samples relating to the secondemitter wavelength plus ambient light, eight more samples related toambient light and so forth until there are eight samples of each emitterwavelength followed by eight samples of ambient light.

Because the signal processing system 334 controls the activation of thelight emitters 301-305, the entire system is synchronous. The data issynchronously divided (and thereby demodulated) into the eight-samplepackets, with a time division demultiplexing operation as represented ina demultiplexing module 421. One eight-sample packet 422 represents thefirst emitter wavelength plus ambient light signal; a secondeight-sample packet 424 represents an ambient light signal; a thirdeight-sample packet 426 represents the attenuated second emitterwavelength light plus ambient light signal; and a fourth eight-samplepacket 428 represents the ambient light signal. Again, this continuesuntil there is a eight-sample packet for each emitter active period withan accompanying eight-sample packet for the corresponding ambient lightperiod. A select signal synchronously controls the demultiplexingoperation so as to divide the time-division multiplexed composite signalat the input of the demultiplexer 421 into its representative subpartsor packets.

A sum of the four last samples from each packet is then calculated, asrepresented in the summing operations 430, 432, 434, 436 of FIG. 6. Itshould be noted that similar operations are performed on the remainingwavelengths. In other words, at the output of the demodulationoperation, five channels are provided in the present embodiment.However, only two channels for two wavelengths are depicted in FIG. 6for simplicity in illustration. The last four samples are used from eachpacket because a low pass filter in the analog to digital converter 356of the present embodiment has a settling time. Thus, collecting the lastfour samples from each eight-sample packet allows the previous signal toclear. The summing operations 430, 432, 434, 436 provide integrationwhich enhances noise immunity. The sum of the respective ambient lightsamples is then subtracted from the sum of the emitter samples, asrepresented in the subtraction modules 438, 440. The subtractionoperation provides some attenuation of the ambient light signal presentin the data. In the present embodiment, it has been found thatapproximately 20 dB attenuation of the ambient light is provided by theoperations of the subtraction modules 438, 440. The resultant emitterwavelength sum values are divided by four, as represented in the divideby four modules 442, 444. Each resultant value provides one sample eachof the emitter wavelength signals at 625 Hz.

It should be understood that the 625 Hz carrier frequency has beenremoved by the demodulation operation 400. The 625 Hz sample data at theoutput of the demodulation operation 400 is sample data without thecarrier frequency. In order to satisfy Nyquist sampling requirements,less than 10 Hz is needed (with an active pulse of about 4 Hz in thepresent embodiment). Accordingly, the 625 Hz resolution is reduced to62.5 Hz in the decimation operation.

FIG. 7 illustrates the operations of the decimation module 402 for thefirst two wavelengths. The same operations are also performed on theother wavelength data. Each emitter's sample data is provided at 625 Hzto respective buffer/filters 450, 452. In the present embodiment, thebuffer/filters are 519 samples deep. Advantageously, the buffer filters450, 452 function as continuous first-in, first-out buffers. The 519samples are subjected to low-pass filtering. Preferably, the low-passfiltering has a cutoff frequency of approximately 7.5 Hz withattenuation of approximately -110 dB. The buffer/filters 450, 452 form aFinite Impulse Response (FIR) filter with coefficients for 519 taps. Inorder to reduce the sample frequency by ten, the low-pass filtercalculation is performed every ten samples, as represented in respectivewavelength decimation by 10 modules 454, 456. In other words, with thetransfer of each new ten samples into the buffer/filters 450, 452, a newlow pass filter calculation is performed by multiplying the impulseresponse (coefficients) by the 519 filter taps. Each filter calculationprovides one output sample for each respective emitter wavelength outputbuffers 458, 460. In the present embodiment, the output buffers 458, 460are also continuous FIFO buffers that hold 570 samples of data. The 570samples provide respective samples or packets (also denoted "snapshot"herein) of samples. As depicted in FIG. 5, the output buffers providesample data for Glucose Calculation Module 408 for two wavelengths.

FIG. 8 illustrates additional functional operation details of theGlucose Calculation module 408. As represented in FIG. 8, the GlucoseCalculation operation accepts packets of samples for each wavelength(e.g., 570 samples at 62.5 Hz in the present embodiment) representingthe attenuated wavelength signals, with the carrier frequency removed.The respective packets for each wavelength signal are normalized with alog function, as represented in the log modules 480, 482. Again, at thispoint, only two channels are illustrated in FIG. 8. However, in thepresent embodiment, five channels are provided, one for each wavelength.The normalization effectively creates an AC/DC normalized signal, thisnormalization is followed by removal of the DC portion of the signals,as represented in the DC Removal modules 484, 486. In the presentembodiment, the DC removal involves ascertaining the DC value of thefirst one of the samples (or the mean of the first several or the meanof an entire snapshot) from each of the respective wavelength snapshots,and removing this DC value from all samples in the respective packets.

Once the DC signal is removed, the signals are subjected to bandpassfiltering, as represented in Bandpass Filter modules 488, 490. In thepresent embodiment, with 570 samples in each packet, the bandpassfilters are configured with 301 taps to provide a FIR filter with alinear phase response and little or no distortion. In the presentembodiment, the bandpass filter has a narrow passband from 3.7-4.3 Hz.This provides a narrow passband which eliminates most noise and leavesthe portion of the signal due to the active pulse. The 301 taps slideover the 570 samples in order to obtain 270 filtered samplesrepresenting the filtered signal of the first emitter wavelength and 270filtered samples representing the filtered signal of the second emitterwavelength, continuing for each emitter wavelength. In an ideal case,the bandpass filters 488, 490 assist in removing the DC in the signal.However, the DC removal operation 484, 486 also assists in DC removal inthe present embodiment.

After filtering, the last 120 samples from each packet (of now 270samples in the present embodiment) are selected for further processingas represented in Select Last 120 Samples modules 492, 494. The last 120samples are selected in order to provide settling time for the system.

The RMS for the samples is then determined for each of the 120-samplepackets (for each wavelength). The process to obtain the overall RMSvalues is represented in the RMS modules 495-499.

The resultant RMS values for each wavelength provide normalizedintensity values for forming equations according to Beer-Lambert's law.In other words, for Beer-Lambert equation

    I=I.sub.o e.sup.-(p1·c.sbsp.1.sup.·ε.sbsp.1.sup.+p1.multidot.c.sbsp.2.sup.·ε.sbsp.2.sup.+etc.)      (3)

then taking the log of operations 480-482:

    ln(I)=ln(I.sub.o)-(pl·c.sub.i ε.sub.1 +pl·c.sub.2 ·ε.sub.1 +etc.)                          (4)

Then performing DC removal though the DC removal operations 484, 486 andBand pass filter operations 488, 490, the the normalized equationbecomes:

    I.sub.normλ =-pl·c.sub.1 ·ε.sub.1 +pl·c.sub.2 ·ε.sub.2 +etc.      (5)

The RMS values (blocks 495-499) for each wavelength provide I_(norm)λfor the left side of Equation (7). The extinction coefficients are knownfor the selected wavelengths.

As will be understood, each equation has a plurality of unknowns.Specifically, each equation will have an unknown term which is theproduct of concentration and pathlength for each of the constituents ofconcern (hemoglobin, oxyhemoglobin, glucose and water in the presentembodiment). Once a normalized Beer-Lambert equation is formed for eachwavelength RMS value (the RMS value representing the normalizedintensity for that wavelength), a matrix is formed as follows:

    I.sub.normλ.sbsb.1 =-(ε.sub.1λ.sbsb.1 ·C.sub.1 +ε.sub.2λ.sbsb.1 C.sub.2 +ε.sub.3λ.sbsb.1 C.sub.3 +ε.sub.4λ.sbsb.1 C.sub.4 +ε.sub.5λ.sbsb.1 C.sub.5) pl       (6)

    I.sub.normλ.sbsb.2 =-(ε.sub.1λ.sbsb.2 C.sub.1 +ε.sub.2λ.sbsb.2 C.sub.2 +ε.sub.3λ.sbsb.2 C.sub.3 +ε.sub.4λ.sbsb.2 C.sub.4 +ε.sub.5λ.sbsb.2 C.sub.5)pl                (7)

    I.sub.normλ.sbsb.3 =-(ε.sub.1λ.sbsb.3 C.sub.1 +ε.sub.2λ.sbsb.3 C.sub.2 +ε.sub.3λ.sbsb.3 C.sub.3 +ε.sub.4λ.sbsb.3 C.sub.4)+ε.sub.5λ.sbsb.3 C.sub.5)pl        (8)

    I.sub.normλ.sbsb.4 =-(ε.sub.1λ.sbsb.4 C.sub.1 +ε.sub.2λ.sbsb.4 C.sub.2 +ε.sub.3λ.sbsb.4 C.sub.3 +ε.sub.4λ.sbsb.4 C.sub.4 +ε.sub.5λ.sbsb.4 C.sub.5)pl                (9)

    I.sub.normλ.sbsb.5 =-(ε.sub.1λ.sbsb.5 C.sub.1 +ε.sub.2λ.sbsb.5 C.sub.2 +ε.sub.3λ.sbsb.5 C.sub.3 +ε.sub.4λ.sbsb.5 C.sub.4 +ε.sub.5λ.sbsb.5 C.sub.5)pl                (10)

where

C₁ =concentration of water

C₂ =concentration of hemoglobin

C₃ =concentration of oxyhemoglobin

C₄ =concentration of Glucose

C₅ =concentration of Glucose

and

ε₁λn =extinction coefficient for water at λn

ε₂λn =extinction coefficient for hemoglobin at λn

ε₃λn =extinction coefficient for oxyhemoglobin at λn

ε₄λn =extinction coefficient for Glucose at λn

ε₅λn =extinction coefficient for Glucose at λn

The equations are solved using conventional matrix algebra in order tosolve for the product of concentration times pathlength for eachconstituent, as represented in the Matrix block 489.

In order to remove the path length term, in the present embodiment whereglucose is desired, a ratio is performed of the product of pathlengthtimes concentration for glucose to the product of pathlength times theconcentration of water as represented in a ratio block 487. Since thepathlength is substantially the same for each wavelength due tonormalization (i.e., taking AC/DC) and due to minimal perturbation(e.g., 10%), the pathlength terms cancel, and the ratio indicates theconcentration of glucose to water (preferably, this is scaled to mg/dL).The glucose concentration is provided to the display 336.

It should be noted that it may also be possible to create an empiricaltable by way of experiment which correlates ratios of one or more of theconcentration times path length terms to blood glucose concentration.

Even with the emitter driver circuit of FIG. 2A discussed above,infrared LEDs with the longer wavelengths are also inherently unstablewith respect to their power transmission. Accordingly, in oneadvantageous embodiment, the instabilities for the source LEDs can becorrected to accommodate for the instabilities depicted in FIG. 2C. Asillustrated in FIG. 2C, two curves are depicted representing transmittedpower over time. A first curve labelled AA represents power transmissionfrom LEDs having wavelengths of 660 nm and 905 nm. As illustrated, theseemitters have relatively stable power transmission over time. A secondcurve labelled BB represents power transmission from an emitter with awavelength of approximately 1330 nm. As illustrated, typical emitters ofthis wavelength have unstable power transmission over time.

Accordingly, in one embodiment, the emitters in the 1300 nm range areselected as with an integrated photodetector. An appropriate laser diodeis an SCW-1300-CD made by Laser Diode, Inc. An appropriate LED is anApitaxx ETX1300T. With such an emitter, a configuration as depicted inFIG. 2B can be used, whereby the internal photodiode in the emitter isalso sampled to detect the initial intensity I_(o) times a constant (α).In general, the signal detected after transmission through the finger isdivided by the αI_(o) signal. In this manner, the instability can benormalized because the instability present in the attenuated signal dueto instability in the emitter will also be present in the measuredαI_(o) signal.

FIG. 2B depicts such an embodiment illustrating only one emitter 301 (ofthe emitters 301-305). However, all or several of the emitters 301-305could be emitters having an internal photodiode. As depicted in FIG. 2B,the emitter 301 has an internal photodiode 301a and its LED 301b. Asdepicted in FIG. 2B, light emitted from the LED 301b in the emitter 301is detected by a photodiode 301a. The signal from the photodiode 301a isprovided to front end analog signal conditioning circuitry 330A. Theanalog signal conditioning circuitry 330A similar to the analog signalconditioning circuitry 330. However, because the photodiode 301a detectsa much stronger intensity compared to the detector 320 (due toattenuation by tissue), different amplification may be required.

After analog signal conditioning in the front end anaolog signalconditioning circuity 330A, the signal from the photodiode 301a isconverted to digital form with an analog to digital conversion circuit332a. Again, it should be understood that the analog to digitalconversion circuit 332a can be the same configuration as the analog todigital conversion circuit 332. However, because the signal from thephotodiode 301a and the detector 320 appear at the same time, twochannels are required.

The attenuated light signal through the finger is detected with thedetector 320 and passed through front end analog signal conditioningcircuit 330 and is converted to digital form in analog to digitalconversion circuit 332, as described in further detail below. The signalrepresenting the intensity of the light transmitted through the finger310 is divided as represented by the division block 333 by the signalwhich represents the intensity of light from the LED 301b detected bythe photodiode 301a.

In this manner, the variations or instability in the initial intensityI_(o) cancel through the division leaving a corrected intensity which isdivided by the constant α. When the log is performed as discussed below,and bandpass filtering is performed, the constant α term is removedleaving a clean signal.

Mathmatically, this can be understood by representing the attenuatedsignal under Beer-Lambert's Law and the signal from the photodiode 301aas αI_(o) as discussed above:

Thus, the signal emerging from the analog to digital conversion circuit332 is as follows:

    I=I.sub.o e.sup.Σ(-ε·c·pl)

Dividing Equation 3 by αI_(o) and simplifying provides the signal afterthe division operation 333: ##EQU1##

Thus providing a normalized intensity signal for the input to thedigital signal processing circuit 334.

FIG. 10 depicts a perspective view of one alternative embodiment of aninflatable bladder sensor 500 which can be used to induce an activepulse in accordance with the teachings of the present invention. Thisinflatable bladder sensor 500 is for a bed-side blood glucose monitor.The inflatable bladder sensor 500 has electrical connections 502 forcoupling the device to the blood glucose system 299.

Typically, the electrical connection 502 carries sufficient conductorsto power the emitters 301-305 and to receive a detector signal from thedetector 320.

The inflatable bladder sensor 500 has a curved upper surface 504 andvertical sides 506. The inflatable bladder sensor 500 also has an fluidpressure supply tube 508. In one advantageous embodiment, the supplytube cycles air into and out of an inflatable bladder within theinflatable bladder sensor 500. The fluid supply tube 508 couples to thebedside glucose monitoring system which is equipped with a cycling pumpto induce pressure and remove pressure from the supply tube 508. In oneembodiment, a pressure relief valve 510 is located on the upper surface504 to allow release of pressure in the inflatable bladder.

FIG. 11 depicts a cross-sectional view along the inflatable bladdersensor 500 of FIG. 10. As depicted in FIG. 11, a human digit or finger512 is positioned inside the sensor 500. The finger 512 is positioned issupported by a pad 514 in the area of light transmission. A flexibleinflatable bladder 516 surrounds the finger proximally from the area oflight transmission. The pad has an an aperture 518 to enable emitters301-305 to provide unobstructed optical transmission to the surface offinger 512.

Surrounded by the padding 514 and opposite the emitters 301-305 is thedetector 320. The detector 320 is positioned within an aperture 520 inthe pad 514 to ensure that photodetector is separated from the finger512. A serpentine arrow is shown extending from the light emitters301-305 to the detector 320 to illustrate the direction of propagationof light energy through the finger 512.

Relief valve 510 enables manual and automatic release of pressure in theinflatable bladder 516. Relief valve 510 has a valve plate 522 which isspring biased to seal an aperture 524. The valve plate is connected torelief valve shaft 526. A valve button 530 is coupled to the valveshaft. The valve shaft extends through a valve housing 530 which forms acylindrical sleeve shape. The valve housing is coupled to the uppersurface 504 of sensor 500. The valve housing has an aperture 523 whichallows air to readily escape from the relief valve. Preferably, therelief valve is designed to ensure that the pressure is not high enoughto cause damage to nerves. Accordingly, if the pressure increases beyonda certain point, the relief valve allows the excess fluid to escape,thereby reducing the pressure to the maximum allowable limit. Suchpressure relief valves are well understood in the art. Relief valve 510could also be a spring-loaded needle-type valve.

FIG. 12 depicts a sectional view along line 12--12 of FIG. 11 toillustrate the state of the sensor 500 when the inflatable bladder 516is deflated. FIG. 12a depicts the same sectional view as FIG. 12 withthe bladder 516 inflated.

With this configuration, the blood glucose system can cycle fluid intoand out of the inflatable bladder 516 at the selected rate to activelyinduce a pulse of sufficient magnitude as discussed above.

Additional Application of Active Pulse

As discussed in the co-pending U.S. patent application Ser. No.08/320,154 filed Oct. 7, 1994, which is incorporated herein byreference, a saturation transform may be applied to each 120 samplepacket. It has been found that a second maxima representing venousoxygen saturation exists in the Master Power Curve during motion of thepatient. In view of this, it is possible to utilize the inducement of apulse disclosed herein through physically perturbing the medium undertest in order to obtain the second maxima in the Master Power Curve, andthereby obtain the venous oxygen saturation if desired. The modulationmay he lower than 10% because hemoglobin and oxyhemoglobinconcentrations are higher than glucose and absorbtion at 660 nm and 905nm are relatively strong. Thus, modulation from 1-5% may provideadequate results.

Although the preferred embodiment of the present invention has beendescribed and illustrated above, those skilled in the art willappreciate that various changes and modifications to the presentinvention do not depart from the spirit of the invention. For example,the principles and method of the present invention could be used todetect trace elements within the bloodstream (e.g., for drug testing,etc.). Accordingly, the scope of the present invention is limited onlyby the scope of the following appended claims.

What is claimed is:
 1. A method of actively varying the attenuation ofoptical radiation due to blood in a fleshy medium comprising the stepsof:transmitting optical radiation through said fleshy medium; detectingsaid optical radiation after attenuation through a field of view throughsaid fleshy medium; generating an output signal indicative of anintensity of an attenuated signal; and applying pressure to said fleshymedium according to a predictable cyclic pattern to influence a cyclicchange in the volume of blood in said fleshy medium in said field ofview to produce no more than 40% modulation in said attenuated signal.